The production of images of subject matter emitting high energy radiation such as X-radiation, gamma radiation and nuclear particles is more complex than the production of images with visible radiation by an optical lens system because there are no materials available having a sufficient refractive power to focus the high energy radiation. While x-rays have been used in the studies of crystals via the well-known Bragg diffraction technique, and while the focussing of soft x-rays by a Fresnel plate of extremely thin elements is disclosed in U.S. Pat. No, 2,679,474 which issued to W. S. Pajes on May 25, 1954, the only structures utilized for the higher energy radiations, particularly gamma rays, for forming visible images are the collimator disclosed in U.S. Pat. No. 3,011,057 which issued to H. O. Anger on Nov. 28, 1961 and the mask disclosed in U.S. Pat. No. 3,748,470 which issued to H. H. Barrett, the inventor of the present invention, on July 24, 1973. Both the collimator and the mask function by blocking certain rays of radiant energy as distinguished from the Bragg diffraction in which rays of radiation are produced in a direction different from an original direction of propagation from a source of such radiation. A mask adapted for examining a point source of high energy radiation is disclosed in U.S. Pat. No. 3,263,079 which issued to L. N. Mertz and N. O. Young on July 26, 1966and in a book entitled "Transformations in Optics" by said L. N. Mertz published in 1965 by John Wiley and Sons, Inc. (the adaptation for point source radiation as distinguished from a continuum of radiation being disclosed at page 91 thereof).
The operation of a collimator is distinctly different from that of a mask in that a collimator permits the passage of radiant energy within the collimator tubular apertures only in a direction parallel to the axis of the tubular aperture while a mask, in which the depth of an aperture is smaller than the width of the aperture, the propagation of radiant energy is permitted within the apertures irrespective of the direction of propagation of the radiant energy. As is disclosed in the aforementioned patent to Barrett, the use of a mask provides a greatly increased effective aperture and the imaging of a source of high energy radiation. This is due to the substantial constriction of the propagation of radiant energy provided by the long tubular apertures of a collimator while the relatively thin mask presents no such constriction. An additional distinguishing feature in the use of collimators as compared to masks, noted in the aforementioned Barrett patent, is the coding or scrambling of the image by the mask which necessitates a subsequent decoding or descrambling to make the image visible.
A problem arises in the use of the mask in that the attainable resolution of the resulting image is dependent on the type of detector utilized. For example, if the detector comprises an array of photomultiplier tubes positioned behind a scintillator crystal in the manner disclosed in the aforementioned patents to Anger and Barrett, the resulting resolution of the image is limited by the number of the photomultiplier tubes even though a very fine mask having many apertures per square inch is utilized. For a given sized mask, an array of 19 photomultipliers provides greater resolution than an array of seven photomultipliers. Greater resolution can be obtained by the use of an image intensifier as the detector as is shown in FIG. 12 of the aforementioned Barrett patent. However, there are situations in which th photomultipliers need be used beause of their greater sensitivity.